The present invention relates to vortex shedding fluid velocity meters.
Fluid flowing past a blunt, non-streamlined obstruction tends to develop vortices at the obstruction. These vortices are detached or "shed" from the obstruction and passed downstream with the fluid flow. In a phenomenon referred to as a "von Karman vortex street," the vortices tend to pass downstream from the obstruction in rows on opposite sides of the obstruction. The vortices in such a vortex street tend to be spaced at predetermined intervals, with the vortices of one row staggered with respect to the vortices in the other row. The distance between adjacent vortices in each row is substantially constant over a fairly wide range of flow rates. Therefore, the number of vortices passing a point downstream from the obstruction per unit time is substantially proportional to the velocity of the flowing fluid and hence proportional to the flow rate. As disclosed in U.S. Pat. No. 3,572,117, it has long been recognized that a flowmeter can be made by providing an appropriate vortex generating obstruction and sensing elements downstream from the obstruction to detect the vortices. The sensors described in the '117 patent detect the localized disturbance in the flow created by each vortex as that vortex passes the sensor.
As the apparatus required for direct measurement of localized flow disturbances is relatively delicate and troublesome, other vortex shedding flowmeters have made with pressure sensors. As described in U.S. Pat. No. 3,972,232, a vortex shedding flowmeter may have a vortex generating obstruction in the form of a generally flat plate, and a barlike tailpiece may be joined to the rear or downstream surface of the plate so that the tailpiece projects downstream from the rear or downstream surface of the plate. The vortices shed alternately from the opposite edges of the plate pass downstream on opposite sides of the tailpiece, and hence create slight disturbances in the fluid pressure on each side of the plate. Because the vortices shed from opposite edges of the plate are staggered with respect one another, they will produce alternating pressure fluctuations on opposite sides of the tailpiece. For any given sensing location along the tailpiece, the pressure at the sensing location will be slightly increased on one side of the tailpiece and slightly depressed on the opposite side of the tailpiece as a vortex passes by the sensing location on one side. When the vortex passes along the other side, the pattern of pressure increase and decrease is reversed. As set forth in the '232 patent, a differential pressure sensor may be mounted to the tailpiece in a predetermined sensing location slightly downstream from the vortex generating element or plate. The sensor taught in the '232 patent includes a laminated, two-layer piezoelectric assembly mounted in a hole passing through the tailpiece from side to side so that the piezoelectric assembly is exposed to fluid pressures on both sides and hence deforms as a unit responsive to differences between the pressures prevailing on opposite sides of the tailpiece. To isolate the piezoelectric element from the flowing fluid, flexible diaphragms cover the hole on both sides, and the spaces between the diaphragms and the piezoelectric element are filled with an inert fluid such as an oil.
The two-layer piezoelectric element is provided with a single set of leads so that one electrical potential across the piezoelectric assembly can be transmitted to an external signal processing circuit. As the piezoelectric assembly bends back and forth as a unit under the influence of the alternating high and low pressures on opposite sides of the tailpiece, the potential of the output signal at the leads varies. The frequency of this variation in theory represents the frequency with which vortices pass the sensing element. Thus, by amplifying this output signal and measuring its frequency, the fluid velocity and hence flow rate can be monitored. In practice, however, the output signal from the piezoelectric assembly contains many spurious fluctuations. These spurious fluctuations can be of about the same magnitude as the periodic variations representing vortex passage. Therefore, the spurious fluctuations can interfere with the frequency measurements and produce substantial error in the velocity or flow readings. Elaborate electronic filtering arrangements have been applied to the output signal from the piezoelectric assembly to counteract these spurious fluctuations. However, even with such arrangements, it is sometimes difficult or impossible to obtain a reliable measurement. This problem is particularly troublesome where the flowmeter is mounted in a noisy environment such as a typical pipeline connected to a pump. Noise created by the pump and propagated through the fluid and through walls of the pipe tends to aggravate the spurious fluctuations in the output signal.
Another serious problem in vortex shedding flowmeters heretofore has been the problem of frequency variation. Thus, the time between shedding of successive vortices tends to vary about a nominal or central value. This causes variations in the frequency of the output signal from the flowmeter. As the nominal frequency or period of the output signal varies about its nominal or central value, it falsely indicates that the fluid flow rate or velocity is also varying. This variation in the frequency or period of the output signal is particularly troublesome where the output signal is fed to an automatic control system or computer.
Accordingly, there have been substantial needs heretofore for improvements in vortex shedding flow meters.